Electrical element



July 28, 1953 L. H. BERKELHAMER ELECTRICAL ELEMENT Filed Dec. 18, 1948IN VEN TOR.

Ww@ fM/Md Patented .uly 28, 1953 UNITED STATES PATENT OFFICE ELECTRICALELEMENT Louis H. Berkelhamer, Chicago, Ill., assigner to David T.Siegel, Glencoe, Ill.

Application December 18, 1948, Serial No. 65,989 s claims. (o1. 2014-64)This invention relates to electrical elements and to methods foreffecting the manufacturing thereof. It concerns particularly electricalcircuit elements of the wire coil resistive type such as wire coilresistors and rheostats.

Wire coil resistive circuit elements, such as resistors and rheostats,as heretofore manufactured have basically been of two types, viz., thosecoated'with vitreous enamel, and those coated with organic lacquers andvarnishes. The lacquer or Varnish coated elements, while relativelyeasily fabricated, have the disadvantage that they are not resistant tohigh temperature, which precludes their use in many circuitinstallations. The vitreous enamel coated elements have high thermalresistivity, and thus are capable of a greater range of use, but require-high firing temperatures to bake the enamel which in turn results incertain marked disadvantages both in the case of resistors and in thecase of rheostats. In the instance of resistors the high firingtemperatures may, particularly in the case of certain alloy wire coilresistors, result in the tempering of the wire or in effecting of otherphysical or chemical changes in the wire which result in a change in theresistance coefficient of the wire body. As va result, the resistorafter the baking of the vitreous enamel no longer has its precalculatedresistive value, which in installations rei-` quiring high accuracyprecludes the use of the resistor or Various testing, selecting, orcompensating operations as will be understood. As to rheostats, theconventional physical form of a rheostat entails the use of a slidablecontactor along an exposed portion of theresistive Wire coil, theremainder of the coil, or parts thereof, being encased within thevitreous enamel material. The baking ofthe vitreous enameLat the highfiring temperatures required, frequently oxidizes the exposed metalportions of the wirefcoil so that these portions must be cleaned afterthe firing process. In instances wherein the coil is formed ofrelatively fine wire, the cleaning whichl 2 stallations, difficultiesare encountered in their attempted use with wire coil resistive circuitelements such as hereinabove mentioned.

In accordance with the present invention, applicant has devised ways andmeans for effecting the satisfactory application of silicone material towire coil resistive circuit elements, such as resistors and rheostats;and has further provided certain materials of addition, with lthesilicone, which it has been found impart to the elements the desired andnecessary characteristics of accuracy, heat resistance, and mechanicalstrength.

It is accordingly an object of the present invention to provide animproved and satisfactory wire coil resistive element, such as a,resistor or rheostat, and to provide satisfactory methods of fabricationof such elements, wherein a silicone material forms a part of theprotective coating for the resistive wire coil.

A further object of the invention is to provide an electrical resistiveelement, embodying a silicone in a protective coating for the wire coil,and wherein the coating is possessed of the requisite and improvedqualities ofinsulation, thermal resistance, and physical strength.

Various other objects, advantages and features of the invention Will beapparent from the following specication when taken in connection withthe accompanying drawings wherein certain preferred embodiments are setforth for purposes of illustration.

In the drawings, wherein like reference numerals refer to like partsthroughout:

Fig. 1 is a perspective view of a Wire coil re-v sistive circuit elementand specifically a resistor, constructed in `accordance with andembodying the principles of the present invention;

Fig. 2 is an enlarged partial longitudinal sectional vieW of theresistor of Fig. l, on the line 2 2 thereof;

Figs. 3 and 4 are views illustrating method steps in the fabrication ofthe resistor, according to the present invention;

Fig. 5 is a top or plan View of a resistive circuit element in the formof a rheostat, constructed in accordance with the invention; y

Fig. 6 is an enlarged sectional view of the rheostat body, forming apart of the rheostat structure of Fig. 5; and' Fig. 7 is a detailsectional View of the rheostat structure of Fig. 5, taken as indicatedby the line `1-l thereof.

Referring more specifically to the drawings, and first to Figs. 1 to 4thereof, in Fig. 1 there is illustrated a wire coil resistor comprisinga body portion l0 provided with terminal lugs l2 and I4 at the oppositeends thereof. The terminal lugs comprise projecting tine portions towhich various circuit elements may be secured, and band portionsembracing the body portion of the resistor and to which the oppositeends of the resistive Wire coil are electrically connected, as will beunderstood.

As best shown in. Fig. 2, the resistor body portion comprises aceramictube I6 upon which is wound the resistive wire coil I8, the variousconvolutions of which are maintained in predetermined spaced relation.In accordance with the present invention the wire coil, its severalconvolutions, and the band portions of the terminal lugs are completelycovered by a coating 2li, thev constituency and manner of application ofwhich will be presently described.

Silicones, in general, are classified as compounds and greases, nuids,varnishes and resins, and rubbers. Of these silicones, some of which maybe properly designated as liquids at room temperatures, the rubbers havethe highest molecular weight. Silicone rubbers are marketed by variousmanufacturers, those made by Dow Corning being designated as SilasticDow Corning designates its Silastic 121-122 as heavy paste, its Silastic123-126 as thin paste, and its Silastic G-160, 161, 167, 180 and 181 asstock.

In compounding and applying the coating 20,

a silicone liquid is given the desired viscosity by mixing With athinner such as toluene or naptha. The silicone liquid, thus treated, isthen mixed with a powdered mineral filler, specifically silica in theparticular embodiment disclosed, and the mix is stirred or beaten toform a suspension. The suspension mix, thus formed, is then placedwithin a suitable tank 22, Fig. 3, as indicated at Z4, and theelectrical unit may be immersed therein, as shown, preferably in amanner so as t0 leave the projecting tine portions of the terminals l2and I4 exposed so as to remain uncoated.

The unit, thus coated, is then placed within an oven 26, as indicated inFig. 4., so as to cure the silicone material. If desired, the unit maybe subjected to several repeated immersion and heating operations so asto buildup the coating as a plurality of layers. For economy the coatingmay be precluded from formation within the inner bore of the tubularceramic body I6 by plugging the ceramic tube or wiping the coatingtherefrom, as desired.

The heat cure temperatureto which the coating is subjected by the oven26, may vary somewhat depending upon the length of time of the heatingoperation, but is in any event within a resistor curing range and timesubstantially above a condensing range but materially below the firingtemperature of vitreous enamel, and suiciently low to preclude achemical or physical' change within the metal of the resistive wire c-oil I8, and resultingly any change in the electrical resistivecoefficient thereof.

While the coating 2G has been illustrated as applied by immersion of theunit within the suspension mix 24, itv will be understood that the mixmay be applied by other physical means such as pouring or otherwise, ifdesired. A method is employed, however, which insures that the miX willpenetrate between the convolutions of the wire coil, as indicated at 23in Fig. 2, so as to insure the insulation of the convolutions from eachother, and so as to permit the coating 20 to adhere directly to theoutersurface of the ceramic tube i6 as well as to the wire surfaces.

. It has been found that the mixing of the silicone liquid with apowdered mineral filler, asVv heretofore described, so as to make asuspension mix, results in an insulation coating which is possessed ofdesired properties of electrical insulation, thermal resistance, andphysical strength; and a coating which is materially superior tosilicone alone particularly as to the thermal resista-nce and physicalstrength characteristics. The coating 20 which comprises a cured mix ofsilicone liquid and the powdered mineral ller is suiiiciently resistantto mechanical abrasion for most installations, and imparts an adequateelectrical insulation and thermal resistivity to the unit; while at thesame time the fabrication and application of the coating as hereindescribed preserves the original predetermined electrical resistivecoefficient of the wire coil so that the resistor in its completelyfabricated form will be possessed of the desired predeterminedresistance value.

In Figs. 5-7 the principles of the invention are illustrated as appliedto an electrical resistive element of the wire coil rheostat type.Referring to Fig. 5, a rheostat is illustrated comprising a ceramic body30, a resistive wire coil 32, and a rotary contactor 34 arranged toselectively engage exposed portions of the wire along the length of thecoil. The opposite ends of the resistive wire coil are secured tosuitable terminal lugs as` indicated at 36 and 38, whereas the rotarycontactor is electrically connected to a companion terminal lug 40, theresistance values between the lugs 38 and 40 thus being selectivelyvariable by the rotation of the contactor 34 in a manner characteristicof rheostat structures.

Referring to Fig. 6, theA ceramic body 33 andA the resistive wire coil32, together with the ceramic ring 33 upon which the coil is wound, areillustrated in enlarged detail. In accordance with the invention thewire coil is embedded within and secured with its mounting ring 33 tothe ceramic body 30 by means of a material mass or coating 42 of thesame character as the coating 20- of the resistor unit previouslydescribed.

In fabricating the rheostat of Figs. 5-7 the mass 42 may be formed bymixing the silicone liquid, the powdered mineral ller, and the thinningsolvent in such proportions of the thinner as to make a relatively heavypaste, the mix being stirred or beaten as previously described to form asuspension of the coating body. The coating or body 42 may be applied toand around the wire coil by suitable means such as a putty knife, or bydipping or the like, the wire coil and its mounting ring 33 then beingdisposed upon the ceramic body 30, as shown, and the coating materialheat cured as hereinbefore described. Prior to the heat curing operationthe excess coating material is wiped from the upper portions of the Wirecoil as indicated in dotted lines at 44 in Fig. 6. The manner in whichthe coating material remains embedded between the convolutions of thewire coil, after the wiping operation, is illustrated in enlarged detailin Fig. 7.

During the curing of the coating 42 within a suitable oven such aspreviously described, the curing temperature is maintained sufficientlylow so as to avoid oxidation ofthe exposed wire coil surfaces asindicated at 46, Fig. 7, which have been wiped clean of the coatingmaterial, whereby the further cleaning or treatment of the exposed wiresurfaces after the heat curing operation is unnecessary. By this meansthe wire coil is at no time subjected to the action of abrasion cleaningmembers or the like so that the spacing of the coil convolutions is notdisturbed and breakage or damage of the wire is precluded. In therheostat structure of Figs. 5-7, the coating material or body 42 intowhich the wire coil is embedded acts not only as a protecting,insulating, and spacing medium for the convolutions of the coil, butalso as an adhesive medium for bonding the wire coil and its mountingring 33 to the main ceramic base 30 of the rheostat structure.

The desirable mechanical, electrical, and thermal characteristics of thecoating 42 have been previously described in reference to the coating 26of the resistor, as will be understood.

It is obvious that various changes may be made in the specific methodsteps and structures as heretofore described Without departing from thespirit of the invention. The invention is accordingly not to be limitedto the specic embodiments shown and described, but only as indicated inthe following claims.

The invention is hereby claimed as follows:

l. A wire coil electrical resistive element comprising a base, aflexible uncovered wire coil wound on and in direct mutual contact withsaid base, and an insulating material comprising a mix of silicone andpowdered mineral iiller forming the immediate insulating coating bothfor the wire coils and for the base and heat cured thereon at atemperature materially below the firing temperature of vitreous enamelto adhere to the coils and to the base therebetween for positioning thecoils and providing physical resistance to abrasion and thermal shock.

References Cited in the ,Kle of this patent UNITED STATES PATENTS NumberName Date 2,021,487 McDonell Nov. 19, 1935 2,040,278 Siegel May 12, 19362,258,218 Rochow Oct. 7, 1941 2,258,220 Rochow Oct. 7, 1941 2,428,053Vasilei Sept. 30, 1947 2,460,795 Warrick Feb. 1, 1949 OTHER REFERENCESBass, Silicones, Proceedings of the I. R. E., July 1945, pages 441-447inclusive.

Knight, Silicones, October 1945, pages 1069- 1073 of Materials andMethods.

Marbaker, Coatings for Wire-Wound Resistors, December 1945, pages329-342 of Journal of the American Ceramic Society (pages 336-337 reliedupon).

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